Replacing or repairing TR-808 switches and pots - and a technique for inkjet printing on satin-silver self-adhesive label material which can be laminated
2015-06-09 Robin Whittle
"Death and taxes are unsolved engineering problems" - Romana Machado.
This page concerns some engineering problems, most of which have now been solved.
This page discusses the repair or
replacement of all the TR-808's
switches and pots, with a final detailed section on the Run/Stop
keyswitch. There was
an unsolved engineering problem regarding
these switches. They fail and are not easily replaced or
repaired. However, there is a promising replacement kit from Social Entropy: http://socialentropy.com
, and now replacement buttons and covers from "hotairparts" in Vienna,
as well as a design for new labels. This section (on Run/Stop and
Tap buttons) includes a description of my inkjet printed matte
aluminium background laminated labeling system
below), which is suitable for front panel prototyping and various other uses.
You may also be interested in my analysis of tact switches for the
The TR-808 Service Manual can be obtained from:
The TR-808 is a mains-operated
device with exposed mains connections
on the power switch and the power supply board. Working on this
machine while it is plugged into a mains socket constitutes a serious
safety hazard. To so so risks serious
injury and DEATH. If you are not an experienced electronic
technician who already knows about mains safety and about static
electricity damage to semiconductor devices, I suggest you not
dismantle this machine at all.
The Volume pot
may become noisy. My fix for this is to de-solder it, dismantle
by bending tabs, and then to clean the contacts with isopropyl alcohol.
I usually spin it around a few times (be sure to do so in the direction
where the wipers are dragged, rather than pushed) and use little strips
of cardboard soaked in
isopropyl alcohol. As far as I know, this fixes the problem for a
few years at least.
Technology Transplant http://www.technologytransplant.com/
sell newly manufactured Volume and Tempo
pots. I haven't seen any trouble with Tempo pots, but they too
could become erratic and benefit from cleaning with isopropyl
alcohol. Likewise the Fine Tempo
Generally, I advise never spraying anything anywhere inside electronic
devices. This is especially so for various fluids which contain
oil. However, if you are not in a position to de-solder,
dismantle, re-assemble and re-solder a noisy pot, it might not be such a
bad alternative to squirt some pure
isopropyl alcohol into the pot
, give it a few dozen turns, and
leave it for an hour or so to dry out. It is hard for me to
imagine this leading to any difficulties, and there's a good chance
this will fix any noise problems, at least for the coming months or
I have not seen any serious trouble with the many small pots
of the TR-808. However, they can become noisy.
Technology Transplant used to sell replacement sets, so it would be
worth contacting them if you want a whole new set. However I
can't imagine why anyone would need such a set, unless they had been
used far more than the pots in TR-808s I have worked on.
In August 2012 I had success systematically cleaning all these pots in the following manner.
- Squirt in a little isopropyl alcohol into all the pots.
- Turn each pot one or two dozen times.
- Squirt in a little more isopropyl alcohol.
compressed air to blow out the isopropyl alcohol, while turning each
pot. This inevitably causes isopropyl alcohol to go elsewhere on
the PCB, in addition to where it went after the first two squirts.
- Leave the whole machine to dry for an hour or so before turning it on again.
The section after this one concerns the
Start/Stop and Tap switches. This section concerns all the others.
I have never seen any problems with the three Rotary
switches. It appears
that ALPS still make this range of switches:
however, it could be tricky to find a supplier with the exact same type
for a one-off order.
There is a little push-button switch for Pattern Clear
. They are a
horrible-feeling switch, with no clicking action at all. I
haven't seen one fail. They are probably easy to replace since switches
such as these, with integral buttons, are widely available.
There is a 3 position Basic Variation
toggle switch and a 2 position I/F
A B switch. Technology Transplant sell a pair of
these, newly manufactured:
There is a 3 position slider switch for Pre Scale
. In my experience,
this type of switch can become erratic due to dust, migrating grease
and/or corrosion. My first approach to dealing with this would be
to squirt some isopropyl alcohol into the switch and move it back and
forth a hundred times or so. It is possible to dismantle them for
cleaning, but it is tricky getting them back together, and I am not
sure that this would be superior to leaving it in-situ and putting some
isopropyl alcohol inside it. Likewise the Sync Input / Output
switch on the
I have never worked on the Power
. Some old Roland mains-powered guitar processing
boxes had a similar switch.
The 16 Tact Switches
particular concern. Much of what follows would also be relevant
to the JP-8
(Jupiter 8) which
uses the same switches. If you have whole sets of these failing,
such as groups of four, then you may have a problem with the printed
ribbon cable connecting to the switch board. This is easily fixed
by replacing the ribbon with direct wiring from PCB to PCB.
The TR-808 tact switches are ALPS SKHCABs. These are a stemless
version of the SKHCAA type used in the TB-303 and TR-606. These
are still produced and are known as SKHCBFA010 for the stemless version
(TR-808 and JP-8) and SKHCBEA010 for the TB-303 type with
stems. These are both "1.27 newton" switches, which means about
124 grams as a force.
I do not recommend using these ALPS switches as replacements. Nor
do I recommend using the ALPS sealed tact switches, since they have
what I consider to be a very poor action. Other people find this
acceptable or desirable in the TB-303, but I can't imagine how they
could be better than the Omron switches discussed below. See ../dfish/tact-switches/
measurements and discussion.
I recommend using Omron B3W-4000 sealed tact switches
to replace the ALPS ones
used in the TR-808. These have a similar feel and are sealed
against liquids and dust. So I expect them to last for many years.
The Omron page for these is:
They are available at Farnell, element14 or whatever they call themselves today: http://www.element14.com
. Mouser has them at a better price, but the illustration on their page (2012-03-12) shows another switch. http://www.mouser.com
One approach was
to buy a complete replacement switch panel from
Technology Transplant. However, in early 2014, this product is no longer available.
These used newly manufactured actuators (black plastic frame)and
buttons. Roland got these black plastic actuators and coloured
buttons from ALPS in the early 1980s. John at Social Entropy
found an obscure ALPS catalogue page which lists the black actuator,
with red LED and tact switch, but without a coloured button, as KHC
11901. Here is the image of the sample-card / catalogue page:
Assuming these are the same as what is used in the TR-808, JP-8 etc,
the switch is a 130 grams force "KHC 10902", which must be the older
name for what I knew as an SKHCAB, the stem-less version of the SKHCAA
(as used in the TB-303) which is now (after ~2009?) known as an
SKHCBEA010. ALPS tact switches, such as the SKHCABs used in the
TR-808, have four little slots in the underside of the body which seem
to be for this style of clip-on actuator.
Searching for "KHC10902" turns up links to service manuals for the Korg
Monopoly and PolySix. I recall these use the same tact switch and
actuator, but the buttons are a different shape and colour. My
hypothesis is that while ALPS supplied the tact switch, actuator and
LED, probably as a single component ready to be inserted in the PCB,
they did not supply the buttons. Therefore, the Red, Orange,
Yellow and White buttons used in the TR-808 (and the JP-8 has Blue)
would have been Roland parts.
Technology Transplant had
good replicas of the actuators and buttons made for their TR-808 switch board. The new LEDs in their boards were
somewhat brighter than the originals, without being excessively bright.
However, the buttons were not absolutely
identical in shape or colour, so it
wouldn't look quite right to install one of these buttons in place of
one which went missing on a TR-808.
A common problem is that a button is missing. This is usually not
just the button which has come adrift, but part of the black plastic
actuator as well. Therefore, the entire actuator needs to be
replaced. The actuator contains a LED and snaps around the switch.
In principle, someone could 3D print
new buttons. However, it would be difficult to get the right
colour, and I expect there would need to be some manual work smoothing
the printed surface to get a nice texture for the part which we touch
with our fingers. 3D printing a black actuator would be easy in
terms of colour, but really tricky in practice, since it uses a thin
piece of plastic as a hinge. This would be really difficult to
reproduce in terms of flexibility and longevity, without the original
injection molded polypropylene (I guess) plastic.
The Technology Transplant TR-808 switch board was a most welcome development, since it
was the only source of replacement actuators and buttons. However,
they use the same kind of unsealed tact switches as Roland used - so
these switches will fail within a few years, due to dust.
It is possible to significantly prolong the life of these unsealed tact
switches by placing a thin strip of Mylar (technically biaxially-oriented polyester/PET) or other plastic between the
tops of the switches and the bottom of the flexible part of the
actuator. I used Mylar film from 3M "Professional Flip-Frame"
overhead Transparency Protectors (RS7110). The strip is a little
wider (left-right) than the whole set of buttons and, I recall, about
20mm long (front-back).
However, I believe it is better to use Omron sealed tact switches
instead of the new switches in these Technology Transplant switch
Here are some photos of the original ALPS tact switches, the actuators
and the buttons.
The actuator has dual thin plastic hinges between the moving part
which attaches to the button and the fixed part. This can fatigue
with excessive force, or perhaps just lots of use. When this
happens, this part of the actuator and the button will come loose and
fall off the machine. Please be careful if you remove the button,
since the force could break the hinge.
There's no way of repairing
this broken actuator, so (sadly, no longer available in 2014) the Technology Transplant replacement boards
were highly desirable. However, if someone installs a complete
new board, this means they have an old board with at least some good
actuators in them. I have one such board and would be interested
in acquiring one or two more, so I can use the original actuators and buttons to
replace occasional broken actuators on machines I am working on. If
you have replaced your original board with a Technology Transport one,
and would like to sell me the original board with most of its buttons
and actuators, please let me know:
This photo shows that the actuator has four little hooks by which it is
aligned with the top surface of the PCB by clipping under the ALPS tact
switch, on the left. There are four slots molded into the switch
body for this purpose. In August 2012, the machine I worked on
had no hooks at the rear of the actuator body. The hooks in the
front (lower on the above photo) are minimal. The above photo
gives the appearance of hooks at the back, but I am not convinced that
these really exist as functional hooks - I suspect they are molding
marks. Perhaps the design was changed. There's no strong
reason to have hooks at the back, since the actuator is held in place
by the LED wires being soldered to the PCB.
The Omron switch on the right has no such slots. Therefore, we
need to file away at these four bottom corners of the Omron switch
body. In August 2012 I decided that we really only need two
slots, on the end of the switch which will be at the front (lower on
the following photo). In 2015 I found it better to hold the switch in a vise and cut the plastic with an knife.
The Omron switch bodies are about 0.1mm higher off the PCB (downwards
in the above photo) than the ALPS switches. Since we rely on
these clips at the front of the switch (bottom of the above photo) to
hold the actuator frame flush with the PCB, it is quite tricky filing
the right amount of material off the switches. Too little and the
clip does not go in at all. Too much and the clip will not hold
against the switch body as intended. For doing this faster I used
a small milling bit from a Dremel tool, affixed to a drill press, with
a guide below. This is tricky, so for a single set of switches I
suggest you just use a file. It may help to cut down or file down
the two locating lugs in the middle of the switch, so you can get a
more shallow angle cut when filing.
Installing Omron sealed tact switches is quite a laborious and tricky task
It is worth it, I believe, since these switches feel good and will
probably last for decades.
It is possible to install new LEDs
while doing the above operation. Modern, more efficient, LEDs may
be excessively bright. This can be fixed by increasing the value
of R60, R62, R64 and R66 on the main board.
Also, it is very tricky to bend the LEDs to the exactly correct dimensions
and insert it into the actuator. The existing LEDs have fine
leads and are held in place by heat-deformed plastic. It is a lot of work
to remove the LEDs,
prepare the actuators for new LEDs, bend the new LEDs' leads correctly
and insert them in the actuators, so when soldered they are all at the
correct height. I think few modern LEDs have fine enough leads to
make this a straightforward task. Generally this would be done by
removing the buttons to give access to the LED with its pre-bend leads
to the holes for these leads. However, it is desirable to do this
work without removing the buttons, so as not to place any extra stress
on the plastic hinges of the actuator. I did figure out a way of
doing this, but it is awkward. The new LED can be put through the
hole of the button, with its leads going straight down through a slot
there, not in the path of the old LEDs leads. Then they leads can
be bent sideways to meet the PCB (this may involve using a Dremel etc.
tool to remove some plastic from the base of the actuator).
However . . . the new LEDs need to be narrow enough to fit, and they
are probably not the right height. So the new LEDs must have one or two
small pieces of tubing put around the leads, to lift the base of the
LED body from the top of the slot in the actuator, while allowing the
downwards force on the leads and so the LED body to press the actuator
down to the PCB. The LED leads and body play a crucial role in
holding the rear end of the actuator to the PCB.
A major advantage of the Technology Transplant panels is that they come
with new LEDs which are moderately brighter than the old ones.
My inclination is not to replace these LEDs. It is a lot of work
and I think the old LEDs work OK.
Installing LEDs in multiple colours would be tricky, especially due to the
fact that they are driven in groups (see the schematic for details)
which means it would be difficult to adjust for different LED
efficiencies except by installing individual resistors in series with
Start / Stop and Tap switches
The above information provides what I
consider to be reasonable ways of coping with failure of most of the
TR-808's pots and switches which are likely to fail.
In 2014 there is an important new development: Social Entropy are developing a replacement kit for the two switches and their white plastic buttons.
This kit is forthcoming - it not yet available for sale. Still in
2015 they are not for sale officially on the website.
Please contact them: http://www.socialentropy.com/quicksilver/contact.html
about these kits.
It consists of a modern keyboard
switch (by Cherry), soldered to a specially designed PCB with pins, which brings it
to the correct height. Since the + pattern of the new switch,
where it connects to the button, does not suit the TR-808's white
plastic buttons, there is a very well crafted, 3D printed (with the tops and sides precisely milled
plastic button in place on the switch. (This can be removed from the switch shafts, but there
is no need to do so.) The idea is that the orange label and the outer transparent
shell from the TR-808 are affixed to the new button.
In February 2014 I installed these in a customer's machine.
(Someone had sprayed oil everywhere and this had made the original
switches completely non-functional. The outcome was a total success.
This approach is clearly superior to any other solution I know
of. I have used them twice since then - likewise, total success.
A machine I worked on in May 2015 had an inactive Run/Stop switch with
its shaft broken off, and so no button, orange label or transparent
shell to go over these. The Tap switch, button, label and shell
was OK. I replaced them both with the Social Entropy kit, plus my
own inkjet printed labels.
The artwork is done with open-source Inkscape. Here it is: TR-808-orange-labels-Start-Stop-Tap-00.svg
Here is a JPG of the design:
For anyone who wants to do their own design, here is a scan of some
original labels, which have not been exposed to light very much: labels-aligned.jpg
The original colour is a vibrant orange, not the faded orange of many labels after 34 years of exposure to light.
Here is the result:
See the next sub-section below for where to get transparent covers / shells / "hulls" for these buttons.
The above Inkscape file can be used to print labels however you
like. For those who are interested in my technique, here it is in
a subsection below. There's no way I could reproduce the vibrant
orange colour of the original labels with the inkjet on this matte
silver material. I suspect no inkjet could do it. The
trouble is that it is not possible to inkjet print on ordinary paper,
such as suitable orange paper, since the ink spreads, or will not sit
in the same place and dry quickly, unless the paper has a special
inkjet compatible coating. Perhaps a good laser printer would
work well on orange paper, in which case it would be necessary to edit
the file to delete the orange rectangles.
3D printed clear covers ("hulls") for these buttons - designed in Vienna
In May 2015, just after I shipped the
TR-808 mentioned above to the customer, I received an email from a
fellow in Vienna who has apparently solved the final problem with these
buttons - he has created both replacement buttons (to match the
original ALPS switch stems) and transparent 3D printed acrylic "hulls"
as he refers to them - covers, or shells - to match these buttons.
I am yet to see one of these, but from all I can tell he has done a good job.
Since these match the original button, and since the Social Entropy
button is machined to be the same shape as the original button, so it
fits the original shells, I think these new shells will work fine with
the Social Entropy buttons.
The buttons and shells can be purchased directly from Shapeways in the USA, who fabricate them.
Here is the link for these covers and buttons:
I will order some and post some pictures here when I get them.
My laminated inkjet printed matte silvery label technique using A-One 29283 material (May 2015)
The following is what I wrote before this SocialEntropy kit became available:
Until June 2015 I thought the material
I print onto was only available via Amazon Japan, via another company
who would buy it on my behalf and ship it to Australia. Now it
seems there are multiple Amazon sellers who will ship to Western
countries, so this makes the whole process a lot easier.
I use an Epson Stylus Pro 3800 with
Ultrachrome K3 pigment (little particles of solid coloured material,
rather than a dye) inks.
The Stylus Pro 3880 is more common, and I guess any of the Epson
printers which use this ink would produce identical results. (The
set of inks for these printers are expensive - they are not domestic
I made a special profile in the Windows driver for printing onto the
A-One-Label matte silver material. It involves using the Photo Black
ink, rather than the Matte Black:
Media Type: Premium Semigloss Photo Paper
Print Quality Level: LEVEL 4
Print Quality: SuperFine - 1440x720dpi (but it might be worth trying
less than this if ink-running is a problem, as mentioned below)
High Speed: Off
Finest Detail: On
Edge Smoothing: Off
I will experiment with a longer than
normal (0 seconds) drying time per print head pass (in "Paper
Configuration) since for solid black areas, and I guess with some other
solid ink patterns, the ink is too wet and tends to run together,
leaving some parts not as black as they should be - a somewhat mottled
effect. This is not a problem for these TR-808 labels.
Obtaining the A-One 29283
The material I print on was, as far as I knew (until early June 2015), only available in
Japan - and at one seller in Singapore. It is a fine plastic material, with a strong (I guess
permanent) self-adhesive backing, with a matte silver finish. It
is specifically intended for inkjet printing. I think that if it
was passed through a laser printer, it would melt - and it is unlikely
that its electrostatic behaviour would enable a laser printer to
deposit toner successfully.
It comes in packs of 10 A4 sheets. The manufacturer's page is:
A-One was purchased by 3M a few years ago. The brand name and product number A-One 29283
Googling for: A-One 29283 was my key to finding this material. Googling, a year or two ago,
I found a handful of small retailers in Japan listed it, but none both
had it in stock and would ship to Australia. So I decided to get
it from Amazon Japan. As far as I knew, it was not possible for
me to do this and have them ship the items to Australia.
Therefore, I used another company to purchase ten packs on my behalf,
and ship them to Australia. This was a lot of trouble to get some
adhesive labels, but it is a superb material, unlike any other I could
find. (If some enterprising person decides to retail these on
eBay or whatever, for direct delivery to Western countries, please let
me know and I will add a link here.)
I used http://www.fromjapan.co.jp/
to purchase and ship the labels. I have only used them for this
transaction, and it went very well. They claim to do about 2000
orders a day, so I think they have a highly efficient operation.
By searching http://www.amazon.co.jp
(there is now an "In English" option) for 29283
I was able to find these labels. In May 2015, a link for one of these is:
The newer packages have a design with five turquoise panels and a
single jar. An earlier package, still used by some sellers, depicts a
In June 2015, thanks to Paul of http://www.punkdisco.co.uk
writing on the Synth-DIY mailing list ( http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
I now know of Amazon sellers, who will deliver A-One 29283 to
customers in Western countries. Paul pointed out that this
material has an Amazon Standard Identifier Number (ASIN) of B0000ACBQC
. He reported that Googling this turned up two such sellers:
, I simply entered B0000ACBQC
into the search box, and then clicked on an option for "More buying
options - new" and found a bunch more sellers who would apparently
deliver to Western countries.
After printing, it is essential to remove all moisture
from the label, before laminating.
I do this with a 240 volt electric frying pan, with a dark grey
non-stick surface, running from a large 120V autotransformer, so it
doesn't heat up too much before the thermostat turns the heater
off. I think it is a 2400 watt frying pan, so at half the
voltage it uses 1/4 the power = 600 watts, so this requires a hefty
I adjust the thermostat carefully and use an infra-red
non-contact thermometer to set the temperature in the 105C to 140C
region. The temperature is not particularly critical, but it
needs to be above the boiling point of water (the Ultrachrome inks are
water-based) and not so hot as to distort the plastic label. I
place the label printed side up in the frying pan and cover it with a
large aluminium pie dish. This means the label will be at exactly
the same temperature as the frying pan, but still allows air to move
through to carry out the moisture. I generally leave the
label there for 15 minutes or so.
A hot air gun, hot air heater or hair dryer would probably work fine too. Likewise an electric oven.
After the label is dried, I laminate it using a standard office supply
shop laminator and lamination pouch. In the case of some labels,
where I hand-write serial numbers and dates, I do this either with a
water-based red fine fibre-tipped pen, or with a solvent-based pen (a
fine Sharpie, which are hard to find now). For the water-based
pen, I do this before the drying stage.
Then the labels can be cut with a ruler and knife (on a glass surface), a rolling wheel paper cutter or whatever.
The next trick, not needed for these TR-808 button labels, is to round
. I do use this for some of the other TR-808
labels For this I use a particular heavy-duty corner cutter. (I
tried a Crop-A-Dile cutter and did not like it.) The small radius
(2mm) corner cutter has no brand name. I got it from the Oregon
though this one
might be good too. This company has a vast array of tools and
lamination supplies, including corner cutters / corner rounders of
The resulting labels are not impervious to water or probably to a
high-humidity environment. The hot-melt glue of the lamination
material seals well on the unprinted A-One label, and (at least for the
Ultrachrome ink I am using) the printed areas too. (I understand
that the adhesive most commonly used for lamination films is ethylene-vinyl acetate
- the same as used hot melt glue gun sticks.) This is quite
an achievement, and it relies on the ink being dry and being well
absorbed by whatever inkjet coating is at the top of the A-One
material. This good bonding enables the creation of labels with
coloured backgrounds right to the edge. It is my impression that
these are generally quite robust labels for general use on electronic
equipment. However, if left out in the rain, some water will get
in the edges and change the satin finish to more of a gloss
finish. Eventually, I think the whole label would degrade.
Ultrachrome inks have a good reputation for light-fastness.
However, I am sure that if they are exposed to lots of light,
potentially over many years, especially with a strong UV component,
that the colours will fade somewhat.
The material is plastic - it is not aluminium. So it will be
relatively easy to cut with a scalpel or craft knife once it is stuck
over slots and holes. I haven't tried this yet.
Keeping dust out of the lamination process is a challenge! It may
be best to do this stage outdoors, away from household fluff and dust.
Likewise keeping dust out of the surface before sticking it down.
One technique is to peel back the backing plastic along one edge of the
label by 1 to 2cm, and fold it back sharply, so the exposed adhesive of
the label can be placed on the surface. Once this is done and the
edge rubbed down firmly, I found I could rub upwards, peeling back the
backing plastic at the same time, so that within a second or two of the
adhesive being exposed, it would be pressed firmly into position.
This also helps reduce the problem of air bubbles being caught under the label.
subsection here concerns the original switches and how they might
be refurbished. In a further subsection below that I consider alternative switches.
All the switches I have had to replace have been of the type shown
immediately below, with a white stem and two melted lugs to hold the
switch together. One problem refurbishing these is how to get the
switch back together.
Another type has a black shaft and side clips to hold the switch
together. If one of these is in a TR-808 (I have not seen one so)
then refurbishing these is a lot easier because the switches can be
snapped apart and back together with ease. If you are fixing a
switch of this latter type, please read the next section on cleaning
the contacts, but it will be obvious how to put your switch back
Refurbishing the existing switches - melted lug sealing
Here are some photo of original switches. It is made by ALPS but I
don't know its part number. I don't know of any replacement
buttons either, but generally these are not damaged, destroyed or lost.
It is possible that later TR-808s had a different type of ALPS switch
with the same dimensions, but a different form of construction, as
described below. For simplicity I will refer to the above type as
the "original switches".
The key point about them is that the top of the body is affixed to the
bottom part, the bottom 3.5mm, by two heat-melted cylindrical
lugs. We need to "drill these out" to dismantle the switch,
though in the past I recall I cut the switch apart with a knife, which was a bad idea.
I used a small Dremel tool to cut away the ends of the melted
lugs. It doesn't take much, just a fraction of a millimetre,
before the switch can be prised apart with a knife-blade in the
split. By cutting a circular path following where the lug has
been melted sideways a little, it is possible to leave the centre of
the lug intact, but still cut away enough of its widened end to make it
possible to prise the switch apart.
The failure mechanism is the lack of conductivity when the conductive
rubber presses against the metal contacts. One of these switches
was 127k ohms when pressed. It needs to be about 10k or less to
activate the circuit.
My usual approach to dealing with rubber contacts is:
- Use some light, clean, fresh, emery paper (dark-grey paper-backed
abrasive sheet) to abrade away the surface of the conductive
rubber. (I don't put any solvents on this, because once when I
did, the rubber became less conductive.) 240 or 400 grit is
probably a good choice.
- Use isopropyl alcohol to clean the metal contacts.
use a very light touch of fine, fresh, emery paper to roughen the
surface of the contact points just a little. 800 grit is probably
a good choice.
In this case, point 3 is a must. The contacts are selectively
gold plated and examination with a stereo zoom microscope revealed a
dark shiny patina on the gold. In both these switches, one
contact was darker than the other, so perhaps there was some kind of
electrical process affecting the deposition of material. In the
one switch on which I tried acetone, before any abrasion, the acetone
appeared to take off the lighter coating on one contact while not
affecting the darker coating on the other
Scratching with a needle cuts through this to the gold and copper
below. The 240 grit emery paper scratched up the surface
revealing (under the microscope at 30x) some copper rather than gold.
This is not ideal, since the copper is subject to corrosion.
Still, it is better than retaining whatever coating had built up on the
gold surface. When I used 800 grit emery paper, very
lightly, under the stereo zoom microscope, I was able to get a gold-looking set of scratches, without obvious copper colour.
An additional step which is possible with these switches is to rotate
the rubber cup so that fresh material is exposed to the raised
contacts. However, once the conductive rubber is sanded, it is
not obvious where it was pressing into the contacts.
Using these techniques I was able to make one of these switches go
consistently below 1k ohms - 800 to 900 ohms. However, the other
was 2k to 3k and went higher, beyond 10k, as I attempted improvements,
such as smoothing down the abraded metal contacts. I assume the
rubber contact material has degenerated in some way, perhaps due to
some solvent which might have been on contact with it. However,
perhaps my abrasion was too rough.
There are some special paints which are intended to fix problems with
these conductive rubber switches. I haven't used any and I don't
at present know which ones are well-regarded, but such a product might
save the second switch. A quick search found this product: http://www.keypadrepair.com
Also, it would be possible to modify the circuit so it was acceptable to have a 10k or higher resistance.
Assuming the switch contacts can be made to work acceptably, this still
leaves a problem with how to fix the switch back together. One
approach is to use a soldering iron to melt the lugs again. This
would probably work, but next time the switch needs to be refurbished,
it will be difficult to do this again.
In the past I tried epoxying these switches together, but that was
after a careless disassembly which I think involved cutting around the
seam of the switch. I buttressed the sides with epoxy with a
filler, going down to the PCB, but still the switches fell apart after
a little use.
Here is a method I devised in March 2012 which I think will work well.
I would solder the twisted joints and probably give them a bit more of a twist to tighten them up further.
This looks a bit rough, but I think it will be strong enough for
long-term use. It is easy to pull it apart in the future.
Although I filed slots in the bottom, the wire is below the base of the
switch so some care or insulation tape or the like will be required
when mounting the switch in the TR-808.
Replacement switch from an MC-4
I was lucky enough to obtain a few of these switches, which have never
been used used, as spare-parts for the keyswitch panel of an MC-4
micro-composer. These are physically compatible with the switches
above, and perhaps they are used on some TR-808s.
They operate on the same principles, so refurbishing the contacts would be along the lines described above.
This one had a resistance of about 500 ohms.
Both types of switch are intended to snap into a punched steel plate
for the purposes of making a numeric, QUERTY or other similar kind of
"keyboard" set of switches.
The sides unclip easily to reveal:
I believe these are a later model, superior, switch to the melted lug
version. The melted lug version used deformation of the silicone
rubber cup to take up the slack of the button moving down past the
point where the contacts closed. This one uses a coil spring, and
I believe this places less stress on the silicone rubber cup.
Does anyone know a source of these switches or anything like them?
Other than the above mentioned Social Entropy kit, which seems to be
an excellent solution, I don't know of any alternative switches.
In a July 2011 discussion:
this switch http://www.smallbearelec.com/Detail.bok?no=36
was suggested, but it only has two pins and is apparently a different height.
It can be safely assumed that the people at Technology Transplant have
a better idea of what is manufactured, and can be manufactured, than
most folks. Yet they have no such switches. I think this is
strong evidence that no such switches are currently being manufactured.
Does anyone have suggestions?
These switches measure 13.5 x 13.5mm in the main body - this is the size of hole they are intended to be mounted in.
The four pins are spaced identically to the ALPS and Omron
switches. I was tempted to think of some adaptation of the
existing body and slider mechanism with an Omron tact switch replacing
the current base of the switch. Another idea is to keep the
existing switch body and base, but have a hole through the bottom of it
and the PCB, with a shaft driving an Omron switch mounted below the PCB.